655 Mpa Grade Martensitic Stainless Steel Having High Toughness and Method for Manufacturing the Same

ABSTRACT

A martensitic stainless steel for inexpensive seamless pipe having 655 MPa yield strength, high toughness and excellent corrosion resistance in high CO 2  environments, and a method for manufacturing thereof is provided. The steel comprises C:0.005-0.05%, Si:0.1-0.5%, Mn: 0.1-2.0% P: −0.005%, S: −0.005%, Cr: 10.0-12.5%, Mo: 0.1-0.5%, Ni: 1.5-3.0%, N: −0.02%, Al: 0.01-0.1%, by weight, while FI value defined by the formula [FI=Cr+Mo−Ni−30 (C+N)] being 5.00 to 8.49, and balance of substantially Fe. The method comprises the steps of reheating the cooled steel at temperatures from 780° C. to 960° C., quenching the reheated steel, and then tempering the quenched steel at temperatures from 550° C. to 650° C.

FIELD OF THE INVENTION

The present invention relates to a martensitic stainless steel as OCTGused in oil wells or gas wells, and to a method for manufacturingthereof, specifically the present invention relates to a martensiticstainless steel for inexpensive seamless pipes having 655 MPa yieldstrength and high toughness, suitable for the uses in high CO₂environments, and a method for manufacturing thereof.

BACKGROUND OF THE INVENTION

In recent years, there has been progressing the exploitation of oilwells and gas wells under various environments, including very deepwells, high temperature and high pressure gas wells, and wells in coldregion. Accordingly, there are also aroused the problems of corrosionunder high CO₂ environments and, for an oil well generating H₂S, ofsulfide stress corrosion cracking (SSC) caused by H₂S. As a result,there is an increased demand of steel pipes having both the 552 MPa orhigher yield strength and the high toughness, necessary for OCTG fordeep wells enduring above-described severe corrosion environments.

Conventional OCTG materials are 410 Steel or 420 Steel specified by theAmerican Iron and Steel Institute (AISI). Although these grades ofsteels are relatively inexpensive and achieve 552 MPa or higher yieldstrength by heat treatment, they do not have satisfactory corrosionresistance and toughness. Furthermore, since these steels contain carbonby about 0.1% or more by weight, they cannot be treated by water-coolingin the manufacturing process, which degrades the production efficiency.

Regarding the martensitic stainless steel having above-describedstrength, high toughness, and high corrosion resistance, and regardingthe method for manufacturing thereof, there are several proposals.

For instance, Patent Literature 1, (Japanese Patent No. 2665009),discloses a steel containing 0.005 to 0.04% C, 12.0 to 17.0% Cr, and 1.5to 6.0% Ni, by weight, and a method for manufacturing thereof. Althoughthe steel has 784 to 1078 MPa yield strength (proof stress), which ishigher than that of general-use 552 MPa grade and 655 MPa grade steels,and although the steel gives favorable corrosion resistance in a 65%nitric acid corrosion test, the steel is not examined for corrosionresistance under high CO₂ environments. Patent Literature 2, (JapanesePatent No. 2091532), discloses a steel containing 0.15% or less C, 9 to16.0% Cr, and 0.2 to 2.5% Ni, by weight, and a method for manufacturingthereof. Since, however, the manufacturing of the steel needs controlledrolling, the method has a problem in the efficiency of manufacturingprocess, and has a limitation on the manufacturing facilities. PatentLiterature 3, (Japanese Patent No. 2995524), discloses a steelcontaining 0.03% or less C, 11 to 17% Cr, and 3.5 to 7.0% Ni, by weight,and a method for manufacturing thereof. Since, however, the steel needsto add 3.5% or more Ni, the steel is not advantageous in economy. PatentLiterature 4, (JP-A-2004-115890), (the term “JP-A” referred to hereinsignifies the “Japanese Patent Laid-Open No.”), discloses a low-Nisteel, containing 0.05% or less C, 10 to 12.5% Cr, and 1.5 to 3.0% Ni,by weight, and a method for manufacturing thereof. The steel is,however, limited to 552 MPa grade strength because these materials asshown above rapidly degrade properties such as toughness and SSCresistance if the strength exceeds 552 MPa grade. Patent Literature 5,(JP-A-2004-99964), discloses a low Ni—Nb steel containing 0.02 to 0.05%Cr 10 to 12% Cr, 1.5 to 3.0% Ni, and 0.005 to 0.10% Nb, by weight, and amethod for manufacturing thereof. The steel is, however, limited to 758MPa grade of strength.

[Patent Literature 1] Japanese Patent No. 2665009

[Patent Literature 2] Japanese Patent No. 2091532

[Patent Literature 3] Japanese Patent No. 2995524

[Patent Literature 4] JP-A-2004-115890

[Patent Literature 5] JP-A-2004-99964

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

As described above, conventional technologies cannot provide OCTG pipessuitable for the use under high CO₂ environments, having 655 to 758 MPayield strength and high toughness, and giving excellent economy. Theinventors of the present invention conducted various studies to solvethe above-described problems of conventional technologies, and inventeda martensitic stainless steel for seamless pipes having 655 MPa gradeyield strength, 200 J or higher satisfactory toughness even at lowtemperature of −40° C., suitable for the uses in high CO₂ environmentsand giving excellent economy, and a method for manufacturing thereof,thus completed the present invention.

Means for Resolution

Specifically, the inventors of the present invention improved corrosionresistance and toughness by suppressing the precipitation of carbidethrough the control of C content to a low level, thereby allowed toapply water-cooling in the manufacturing process. With the limitation ofC content, the inventors of the present invention found that thecorrosion resistance is maintained to the level of conventional steelseven when the Cr content is reduced to below 13% which is the content inconventional 420 Steel and the like. Furthermore, along with thelimitation of Cr content, the Ni content is reduced to about 2% toreduce the cost. In addition, the inventors of the present inventionfound that the addition of small amount of Mo provides the steel withstable toughness durable for the use in cold: region, for example, at−40° C.

In the course of completing the present invention, the inventors of thepresent invention investigated the optimum balance of chemicalcompositions and heat treatment conditions in terms ofstrength-toughness-corrosion resistance under high CO₂ environment byvarying the heat treatment conditions of quenching and tempering afterthe hot-rolling of steel having different chemical compositions. Throughthe investigation, a martensitic stainless steel for seamless pipeshaving 655 MPa grade of yield strength suitable for the use under highCO₂ environments was obtained by controlling the chemical compositionsand the heat treatment conditions in a specific range according to thepresent invention.

The 655 MPa grade martensitic stainless steel having high toughness andthe method for manufacturing thereof according to the present inventionhave been completed on the basis of the above-described findings, andthe essence thereof is the following.

A 655 MPa grade martensitic stainless steel having high toughnessaccording to a first aspect of the present invention is a steel havinghigh toughness, comprising C:0.005-0.05%, 51:0.1-0.5%, Mn: 0.1-2.0%, P:−0.05%, S: −0.005%, Cr: 10.0-12.5%, Mo: 0.1-0.5%, Ni: 1.5-3.0%, N:−0.02%, Al: 0.01-0.1%, by weight, while FI value defined by the formula[FI=Cr+Mo−Ni−30(C+N)] being 5.00 to 8.49, and balance of substantiallyFe.

A 655 MPa grade martensitic stainless steel having high toughnessaccording to a second aspect of the present invention is a steel havingthe chemical compositions of the first aspect thereof, furthercomprising at least one chemical composition selected from the groupconsisting of Cu: −0. 5%, Nb: −0.05%, V: −0.1%, B: −0.005%, Ca: −0.005%,by weight.

A method for manufacturing a 655 MPa grade martensitic stainless steelhaving high toughness according to a third aspect of the presentinvention, comprising the steps of: hot-rolling a steel comprising0.005-0.05% C, Si:0.1-0.5%, Mn: 0.1-2.0%, P: −0.05%, S: −0.005%, Cr:10.0-12.5%, Mo: 0.1-0.5%, Ni: 1.5-3.0%, N: −0.02%, Al: 0.01-0.1%, byweight, while FI value defined by the formula [FI=Cr+Mo−Ni−30(C+N)]being 5.00 to 8.49, and balance of substantially Fe; cooling thehot-rolled steel; reheating the cooled steel at temperatures from 780°C. to 960° C.; quenching the reheated steel; and then tempering thequenched steel at temperatures from 550° C. to 650° C.

A method for manufacturing 655 MPa grade martensitic stainless steelhaving high toughness according to a fourth aspect of the presentinvention, comprising the steps of: hot-rolling the steel having thechemical compositions of the third aspect thereof further comprising atleast one chemical composition selected from the group consisting of Cu:−0.5%, Nb: −0.05%, V: −0.1%, B: −0.005%, Ca: −0.005%, by weight, coolingthe hot-rolled steel; reheating the cooled steel at temperatures from780° C. to 960° C.; quenching the reheated steel; and then tempering thequenched steel at temperatures from 550° C. to 650° C.

DETAILED DESCRIPTION FOR CARRYING OUT THE INVENTION

The following is the description about the reasons for limiting thechemical compositions and the manufacturing conditions according to thepresent invention to above-described ranges. The percentage ofindividual chemical compositions signifies percentage by weight.

C: 0.005 to 0.05%

Carbon increases the strength of steel by enhancing solid solution, byhardening the transformed martensite, and by precipitation hardening ascarbide. If the C content is in this range, the effect of increasing thestrength of steel is attained, and both the toughness and the corrosionresistance are maintained to a favorable level. In particular, decreasein the C content to the level of the present invention suppresses theprecipitation of carbide in the steel so that the corrosion resistanceunder high CO₂ environments becomes high.

Si: 0.1 to 0.5%

Silicon inevitably exists in steel as a deoxidizing element. If the Sicontent is in this range, the deoxidizing effect is attained and thetoughness is maintained.

Mn: 0.1 to 2.0%

Manganese inevitably exists as a deoxidizing element in steel, similarwith Si. If the Mn content is in this range, the deoxidizing effect isattained and the toughness is maintained.

P: 0.05% or Less

Since phosphorus is an impurity element and induces degradation oftoughness, lower P content is more preferable. If the P content is inthis range, the degradation of toughness is suppressed.

S: 0.005% or Less

Sulfur is an impurity element similar with P, and induces degradation oftoughness and hot-workability. Accordingly, lower S content is morepreferable. If the S content is in this range, the degradation oftoughness and hot-workability is suppressed.

Cr: 10.0 to 12.5%

Chromium has an effect of improving the corrosion resistance. If the Crcontent is in this range, satisfactory corrosion resistance is attainedeven when C content is low, from 0.005% to 0.05%. If the Cr contentexceeds this range, the effect of increasing the corrosion resistancesaturates, which is not advantageous in terms of economy. Preferably theCr content is in a range from 10.0% or more to less than 12.0%, and morepreferably from 11.0% or more to less than 12.0%.

Mo: 0.1 to 0.5%

Molybdenum has effects to limit the precipitation sites and the kinds ofprecipitates and to improve the toughness. In particular, if thequantity of precipitate of Cr₂(C, N) and M₇C₃ is less than the quantityof precipitate of M₂₃C₆, effective increase in toughness is available.If the Mo content is in this range, the effect of improving toughness isattained. Excess addition of Mo above the range is unfavorable in termsof δ-ferrite formation and of economy. Preferably the Mo content is from0.15 to 0.40%.

Ni: 1.5 to 3.0%

Nickel has an effect to improve the corrosion resistance and thetoughness. If the Ni content is in this range, the δ-ferrite formationis suppressed, which gives favorable hot-workability. Excess addition ofNi above the range is not advantageous because the effect of improvingthe toughness saturates. Preferably the Ni content is in a range from,2.0 to 3.0%.

N: 0.02% or Less

Nitrogen has an effect of strength-increase by the enhancement of solidsolution and by the precipitation hardening. Excess addition of N abovethe range induces binding N with V, Nb, Ti, and the like to form coarseprecipitates, which degrades the toughness and hot-workability.Therefore, excess addition of N is not advantageous.

Al: 0.01 to 0.1%

Aluminum inevitably exists in steel as a deoxidizing element. If the Alcontent is in this range, the effect of deoxidization is attained, andthe formation of AlN which precipitates in grain boundaries to decreasethe grain boundary strength is suppressed, thus to degrade thetoughness.

FI value: 5.00 to 8.49%

The FT is defined by the following formula:

FI=Cr+Mo−Ni−30(C+N)

The FI is a parameter of formation of δ-ferrite. The coefficient isselected in accordance with Schaefler diagram. If the FI value is notmore than 8.49%, toughness becomes favorable because the formation ofδ-ferrite is suppressed.

Balance of the components is substantially Fe. The phrase “substantiallyFe” referred to herein means that the steel of the present invention mayallow the existence of gas components of O and H, and impurities such asSn, As, and Sb, both of which are inevitably contained in the steelduring melting and refining process in the range not affect the purposeof the present invention.

According to the present invention, the steel may further contain atleast one chemical composition selected from the group consisting of Cu,Nb, V, B, and Ca to improve strength, toughness, corrosion resistance,and hot-workability. The reasons to limit these chemical compositionsare described in the following.

Cu: 0.5% or Less

Copper has an effect of increasing the corrosion resistance. If the Cucontent is in this range, no problem of degradation of hot-workabilityand other characteristics occurs.

Nb: 0.05% or Less

Niobium refines austenitic grains by the Nb-carbide precipitation duringquenching thus to improve the toughness, and can increase the strengthby fine Nb-carbide precipitated during tempering. If the Nb content isin this range, the increase in the strength by the precipitation ofNb-carbide is controlled to an appropriate range.

V: 0.1% or Less

Vanadium has an effect of forming nitride with N, thus increasing thestrength. If the V content is in this range, the effect of increasingthe strength does not saturate, and the degradation of toughness causedby the formation of coarse precipitates is not induced, so the strengthincreases without degrading the toughness.

B: 0.005% or Less

Boron has an effect of grain boundary strengthening. If the B content isin this ranger no low-melting compound is formed at grain boundaries sothat the grain boundaries are strengthened while maintaining thehot-workability.

Ca: 0.005% or Less

Calcium has an effect to control the morphology of manganese sulfideinclusion and to improve the toughness and the corrosion resistance. Ifthe Ca content is in this range, the formation of Ca-based precipitateis suppressed, thereby to improve the toughness and the corrosionresistance.

The manufacturing conditions are described below.

Temperature of heating for quenching: 780° C. to 960° C.

If the heating temperature for quenching is in this range, a fullyaustenitic single-phase microstructure is attained during heating sothat the succeeding cooling (quenching) gives martensite-single phasemicrostructure to provide a stable quenched microstructure, as well assuppressing the formation of coarse austenitic grains, thus attainingfavorable toughness.

Tempering Temperature: 550° C. to 650° C.

Since the steel according to the present invention has high strength andinsufficient toughness in as-quenched state, appropriate tempering isrequired. If the tempering temperature is in this range, desirablestrength is attained and the toughness becomes favorable.

The martensitic stainless steel according to the present invention maybe prepared by any melting and refining process such as LD converter orelectric furnace, which can control the chemical compositions within therange of the present invention. When the steel is used as OCTG, thesteel is formed into billet or other required shape by casting orrolling thereof, and then is subjected to a process such as piercingusing a piercer with extrusion stem or a piercing mill with inclinedroll, or rolling to form seamless steel pipes followed by specified heattreatment.

The martensitic stainless steel according to the present invention isapplicable to other usage other than OCTG. For example, the steel may beused as transportation steel pipes such as line pipes. In that case, theingoted steel is formed into slab shape by casting or rolling, and it isrolled into steel plate using a plate mill or a hot-strip mill, and thenis subjected to a specified heat treatment, followed by welding tomanufacture the steel pipes. Alternatively, the rolled steel plate maybe formed into steel pipe by welding thereof, followed by a specifiedheat treatment such as quenching and tempering.

The martensitic stainless steel according to the present invention issubjected to reheating and quenching after hot-rolling. If, however, adirect quenching apparatus which can apply direct quenching to the steelafter hot-rolling is available, direct quenching may be applied insteadof reheating and quenching, followed by specified tempering.

EXAMPLES

Table 1 shows the chemical compositions of the working examples (Nos.1-17) according to the present invention. Table 2 shows the results ofmechanical properties thereof. Table 3 shows the chemical compositionsof the comparative examples (Nos. 18-35). Table 4 shows the results ofmechanical properties.

TABLE 1 Quenching Tempering Chemical Compositions (wt %) Temp. Temp. No.C Si Mn P S Cr Mo Ni N Al Others FI, % (° C.) (° C.) Steels of 1 0.0330.19 0.20 0.011 0.003 11.8 0.19 2.65 0.014 0.051 — 7.93 850 650 thepresent 2 0.033 0.19 0.20 0.011 0.003 11.8 0.19 2.65 0.014 0.051 — 7.93800 650 invention 3 0.033 0.19 0.20 0.011 0.003 11.8 0.19 2.65 0.0140.051 — 7.93 800 625 4 0.033 0.20 0.21 0.010 0.003 11.8 0.37 2.63 0.0150.049 — 8.10 850 650 5 0.033 0.20 0.21 0.010 0.003 11.8 0.37 2.63 0.0150.049 — 8.10 800 650 6 0.033 0.20 0.21 0.010 0.003 11.8 0.37 2.63 0.0150.049 — 8.10 800 625 7 0.029 0.20 0.21 0.006 0.002 11.6 0.18 2.20 0.0150.040 — 8.26 850 625 8 0.029 0.20 0.21 0.006 0.002 11.6 0.18 2.20 0.0150.040 — 8.26 800 625 9 0.029 0.20 0.21 0.006 0.002 11.6 0.18 2.20 0.0150.040 — 8.26 800 600 10 0.032 0.20 0.21 0.010 0.002 11.8 0.38 2.32 0.0150.048 — 8.45 850 650 11 0.032 0.20 0.21 0.010 0.002 11.8 0.38 2.32 0.0150.048 — 8.45 800 625 12 0.032 0.20 0.21 0.010 0.002 11.8 0.38 2.32 0.0150.048 — 8.45 800 600 13 0.032 0.20 0.21 0.008 0.002 11.6 0.21 2.38 0.0150.055  0.3Cu 8.02 800 625 14 0.032 0.20 0.21 0.008 0.002 11.6 0.21 2.380.015 0.055  0.02Nb 8.02 800 625 15 0.033 0.20 0.21 0.006 0.002 11.60.23 2.35 0.015 0.052  0.07V 8.04 800 650 16 0.033 0.20 0.21 0.006 0.00211.6 0.23 2.35 0.015 0.052 0.003B 8.04 800 625 17 0.033 0.20 0.21 0.0060.002 11.6 0.23 2.35 0.015 0.052 0.003Ca 8.04 800 625

TABLE 2 Yield strength vE-40 Corrosion rate Total No. (MPa) (J) (mm/y)Evaluation Steels of 1 668 226 0.05 ∘ the present 2 680 274 0.05 ∘invention 3 713 253 0.03 ∘ 4 676 265 0.03 ∘ 5 668 281 0.04 ∘ 6 717 2670.03 ∘ 7 676 213 0.06 ∘ 8 669 277 0.07 ∘ 9 690 269 0.06 ∘ 10 679 2760.06 ∘ 11 694 302 0.04 ∘ 12 724 292 0.05 ∘ 14 690 243 0.05 ∘ 15 703 2550.05 ∘ 16 712 247 0.05 ∘ 17 699 271 0.03 ∘ 18 710 249 0.04 ∘

TABLE 3

Hatched and Boldface indicate without the range of the present invention

TABLE 4

Hatched and Boldface indicate without the range of the present inventionNA: not determined

All of the steels of working examples according to the present inventionand the steels of comparative examples were vacuum melted using alaboratory furnace, and the obtained ingots were hot rolled to 12mm-thick plates. Each of plates was heat-treated, and was tested todetermine strength, toughness, and corrosion resistance. Regarding thestrength, round bar samples of ASTM Type F were cut from the centerportion in the thickness direction of the plate, and the samples weretested by tensile tester to determine the yield strength. As for thetoughness, full-size V-notch Charpy test samples were cut from thecenter portion in the thickness direction of the plate, and the sampleswere tested by impact tester at −40° C. to evaluate the absorbed energy.For the corrosion resistance, samples were immersed in a 10% NaClaqueous solution equilibrated with 30 bar carbon dioxide gas at 100° C.for 336 hours, and the corrosion loss was evaluated.

The acceptable target values were 655 to 758 MPa of yield strength forthe strength, 200 J or higher absorbed energy (vE⁻⁴⁰) at −40° C. for thetoughness, and 0.3 mm/year or less of corrosion rate for the corrosionresistance.

In Tables 1-4, the Steels Nos. 1-17, which were within the range of thepresent invention in terms of both the chemical compositions and themanufacturing conditions, were proved to have satisfactory strength,toughness, and corrosion resistance. On the other hand, the Steels Nos.18-35, which were outside the range of the present invention in terms ofchemical compositions or manufacturing conditions, failed to achieve thetarget properties of either the strength or the toughness, or both ofthem. In particular, the Steels Nos. 26-29 which contained Mo outsidethe range of the Mo content according to the present invention could notachieve sufficient absorbed energy at −40° C. or sufficient strengtheven if they were subjected to heat treatment within the range of thepresent invention.

EFFECT OF THE INVENTION

The present invention improves the properties required for the seamlesspipes by specifying the chemical compositions and the manufacturingconditions. As a result, the present invention provides a martensiticstainless steel of 655 MPa grade for seamless steel pipes suitable foruse under high CO₂ environments and having high toughness, with lowcost.

1. A 655 MPa grade martensitic stainless steel having high toughness,comprising C:0.005-0.05%, Si:0.1-0.5%, Mn: 0.1-2.0%, P: −0.05%, S:−0.005%, Cr: 10.0-12.5%, Mo: 0.1-0.5%, Ni: 1.5-3.0%, N: −0.02%, Al:0.01-0.1% by weight, while FI value defined by the formula[FI=Cr+Mo−Ni−30(C+N)] being 5.00 to 8.49, and balance of substantiallyFe.
 2. The 655 MPa grade martensitic stainless steel having hightoughness according to claim 1, further comprising at least one chemicalcomposition selected from the group consisting of Cu: −0.5%, Nb: −0.05%,V: −0.1%, B: −0.005%, Ca: −0.005%, by weight.
 3. A method formanufacturing a 655 MPa grade martensitic stainless steel having hightoughness, comprising the steps of: hot-rolling a steel, comprisingC:0.005-0.05%, Si:0.1-0.5%, Mn: 0.1-2.0%, P: −0.05%., S: 0.005%, Cr:10.0-12.5%, Mo: 0.1-0. 5%, Ni: 1.5-3.0%, Ni: −0.02%, Al: 0.01-0.1%, byweight, while FI value defined by the formula [FI=Cr+Mo−Ni−30(C+N) ]being 5.00 to 8.49, and balance of substantially Fe, cooling thehot-rolled steel; reheating the cooled steel at temperatures from 780°C. to 960° C.; quenching the reheated steel; and then tempering thequenched steel at temperatures from 550° C. to 650° C.
 4. The method formanufacturing the 655 MPa grade stainless steel having high toughnessaccording to claim 3, comprising the steps of: hot-rolling the steelfurther comprising at least one chemical composition selected from thegroup consisting of Cu: −0.5%, Nb: −0.05%, V: −0.1%, B: −0.005%, Ca:−0.005%, flyweight; cooling the hot-rolled steel; reheating the cooledsteel at temperatures from 780° C. to 960° C.; quenching the reheatedsteel; and then tempering the quenched steel at temperatures from 550°C. to 650° C.